Question

我有两个模型：

class Post < ApplicationRecord
  has_one :metric, dependent: :destroy
end

class Metric < ApplicationRecord
  belongs_to :post
end

由于某种原因，我不完全了解，因此无法通过帖子创建新指标：

> post = Post.first 1
> post.metric # => Nil
> post.metric.create # NoMethodError (undefined method `create' for nil:NilClass)

为了使这项工作有效，我是否需要声明其他任何模型？

Answer 1

你在做什么

post = Post.first 1
# You don't tell us what this evaluates to, but I'm assuming a post as the #metric call works

post.metric # => Nil
# You have nil

post.metric.create
# Now you're calling the create method on nil.

您需要做的是

Metric.create(post_id: Post.first.id, etc_attribute: etc_value, ...)

编辑：7urkm3n在他的评论中也说过-build_metric和create_metric是利用Rails魔术的更干净的解决方案。

Answer 2

您不能在nil类上调用create方法。

Active Record Associations DOC

post = Post.first 1
post.build_metric # will be automatically assigned post_id to metrics but not saved
post.save
#or
post.create_metric # will be automatically assigned post_id to metrics and saved

替代解决方案：

Metric.create(post_id: post.id)

无法访问has_one关系中的道具

2 个答案: